Prior art vacuum cleaners include a bypass air passage providing communication between a suction side of a fan motor and the atmosphere in bypassing relation to a filter, and a valve device mounted in the bypass air passage and arranged to open only when the filter is clogged or the pressure at the suction side of the fan motor is below a predetermined value when excessive amount of dust is collected.
With such vacuum cleaners, when the load imposed on the fan motor exceeds a certain value, the valve device operates to divert ambient air through the bypass air passage to the fan motor to prevent it from burning out.
Some vacuum cleaners also have a sound producing unit, such as a whistle, disposed in the bypass air passage and actuatable by an air flow to give off an alarm sound when the filter is clogged.
When the vacuum cleaner is used to clean tight places, such as corners of a room or narrow gaps behind pieces of furniture, the vacuum cleaner is equipped with a crevice tool having a constricted suction opening. Since the suction opening is narrower than that of an ordinary floor nozzle, the crevice tool offers a substantial amount of flow resistance to suction air as if the filter were clogged. Therefore, the sound producing unit would be activated to give off an alarm sound during normal cleaning operation. Such alarm sound could be prevented by forcibly closing the bypass air passage while the crevice tool is in use. However, the closed bypass air passage would block air supply to the fan motor which is being driven under full load. This results in overheating burning out of the fan motor.